1. Field of the Invention
This invention relates to BaFCl:Eu phosphors and to their preparation.
2. Background
The use of europium-doped barium fluorochloride (BaFCl:Eu) as a luminescent material in x-ray intensifying screens is well-known. The emission of light occurs in the violet and ultraviolet region of the spectrum, which is especially convenient for use with x-ray films, and it occurs with substantially greater efficiency than is obtained with calcium tungstate, which has been widely employed in x-ray screens. BaFCl:Eu also exhibits excellent stopping power for x-rays.
Various processes are known in the art for preparing BaFCl:Eu phosphor which is useful in x-ray intensifying screens, for example, U.S. Pat. No. 3,988,252 and U.S. Pat. No. 4,080,306. The processes of the art provide BaFCl:Eu particles that are flat and plate-like. FIG. 1 shows the scanning electron micrograph (S.E.M.) of milled particles of U.S. Pat. No. 3,988,252. Although the process of U.S. Pat. No. 4,080,306 provides particles with rounded edges and surfaces, the particles retain the flat plate-like habit, as shown by the S.E.M. in FIG. 2.
Plate-like particles can cause difficulties in x-ray intensifying screen fabrication. Although from the standpoint of screen-performance, it is desirable to maximize the phosphor content in the screen, generally, phosphor particles having a plate-like habit require a higher binder to phosphor ratio than those that are more spheroidal. Furthermore, screens made with plate-like phosphor particles are more prone to bubble or blister when the protective layers are applied, and this results in an unacceptable screen. The plate-like phosphor particles can be milled and classified to obtain particles that are less plate-like and that are within the proper size range. However, such milling damages the surfaces of the crystallites, introduces impurities due to wear of grinding medium, and generates subsize fine particles which represent material loss, and the product consists of particles with a wide range of sizes and shapes.
It is an object of this invention to provide BaFCl:Eu phosphor particles which are spheroidal in shape. Another object is to provide such spheroidal particles having a narrow size distribution. Still another object is to provide a process for preparing such spheroidal particles. Other objects will become apparent hereinafter.